This image provided by Shaam News Network on Thursday, Aug. 22, 2013, has been authenticated based on its contents and other AP reporting. It purports to show bodies of victims of an attack on Ghouta, Syria AP

This image provided by Shaam News Network on Thursday, Aug. 22, 2013, has been authenticated based on its contents and other AP reporting. It purports to show bodies of victims of an attack on Ghouta, Syria AP

The Obama administration, along with key ally Britain, strongly suggested that a chemical attack was the reason for scores of civilian deaths in Syria this week but continued to push for a U.N. investigation before committing to any punishment for President Bashar Assad’s regime.

Whether the death toll is more than 1,000, as Syrian opposition figures claim, or in the low hundreds, as visible in photographs and videos from the scene in an eastern suburb of Damascus, officials sounded increasingly confident that some type of chemical agent was used.

So far, however, a U.N. inspection team that’s now inside Syria hasn’t been granted access to examine the site in Ghouta, leaving the United States and its partners caught between global outrage over the disturbing images and the need for conclusive evidence before vowing a response. If confirmed, it would be the most flagrant violation yet of President Barack Obama’s “red line,” which already was breached with small-scale chemical attacks before this week’s mass-casualty assault, according to the administration.

Obama defended his wait-and-see approach to Syria, though he suggested that the window for U.S. action was narrowing. In an interview on CNN, he called the latest potential chemical weapons attack “clearly a big event of grave concern” and said that U.S. officials are pushing for action from the U.N. – and for the Syrian government to allow investigators access to the site.

A U.S. defense official, who spoke on the condition of anonymity in order to speak candidly, told McClatchy late Friday that a fourth Navy destroyer equipped with Tomahawk cruise missiles was moved closer to Syria on Thursday.

But the official said any reports of preparations for a possible missile attack were “way overstated,” and he said the three warships already in place in the eastern Mediterranean Sea had more than enough firepower to launch an attack were one to be made.

Senior U.S. officials are in close touch with their counterparts in allied European and Arab nations to discuss options for a response, presumably some form of military intervention. The administration has garnered scorn from Syrian opposition figures and rebels for waiting out the conflict, now in its third year. More than 100,000 have been killed, and it is now a bloody fight between rebel forces with al Qaida backing and a regime that’s supported by Iran, Russia and the Lebanese militants of Hezbollah.

Obama warned in the CNN interview that the notion that the U.S. could solve a “sectarian, complex” conflict like Syria is “overstated.” But he added that “when you start seeing chemical weapons used on a large scale, that starts getting to some core national interests.”

Jeffrey White, a defense fellow at the Washington Institute for Near East Policy, a research center, said that if the chemical weapons story holds up to scrutiny, Obama will be under intense pressure to respond with some form of direct military action, short of sending in forces. But he said that so far the administration has resisted such calls to strike, and there’s no guarantee that would change even with a solid finding of chemical weapons use.

“Based on what they’ve done so far, I can’t say they will (take military action),” White said. “Proposals have gone to the White House and died there, fast.”

British Foreign Secretary William Hague said Friday that the attack “is not something that a humane or civilized world can ignore.” But he echoed Obama in saying that the international priority was getting the U.N. team into the area to investigate. If Assad doesn’t grant access within days, Hague said, the evidence would deteriorate and then “we will need to be ready to go back to the Security Council to get a stronger mandate.”

Even Russia, which continues to support Assad politically and with weapons, is urging the regime to allow the inspectors access, according to statements from Moscow. The Syrian government has denied the chemical weapons allegations, but it hasn’t made public statements about whether it would allow the U.N. inspectors into Ghouta.

“Right now, we just see the Assad regime that is standing in the way of that investigation, and that’s something that we’re quite concerned about and that puts the Assad regime not just at odds with the United States and with the U.N. investigative team that’s there, but with the broader international community,” White House spokesman Josh Earnest told reporters Friday.

When asked whether Obama would be considering unilateral action, Earnest said that the president is “committed to working with the international community on this” and noted the “encouraging statement from the Russians.”

Assad isn’t the only holdup to an investigation; another potential snag is the U.N.’s own risk assessment about traveling into such a volatile area. Kevin Kennedy, a retired Marine colonel who’s in charge of U.N. security, told reporters Friday that his office hadn’t yet determined whether it was safe enough for the inspectors to visit Ghouta, according to The Cable blog on the website of Foreign Policy.

“There’s places in Syria we’ve not gone to for months simply because it’s just not safe to go and we can’t mitigate the risk,” Kennedy told a small group of reporters at the U.N., according to the report.

Army Gen. Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, is scheduled to attend a high-level meeting in Jordan next week to discuss the security situation in the region and the ongoing crises in neighboring countries, particularly Syria. Top military leaders from Britain, Saudi Arabia, Turkey, Qatar, France, Germany, Italy and Canada also will attend the meeting, which has been planned since June.

From afar, an increasing number of world experts on chemical weapons are more confident than in previous alleged attacks that such arms were used in Ghouta. Because they don’t have firsthand access to witnesses, soil samples and other evidence, the analysts are carefully studying a flurry of amateur videos posted online that show shrapnel from the purported delivery systems and dozens of people suffering symptoms that are consistent with the effects of nerve agents.

“The exact nature of the agent or agents is impossible to determine from the pictures or film footage, even though a minority of people seem to show outward signs of exposure to a neurointoxicant,” a family of compounds that includes bug sprays at one end of the spectrum and the warfare agents sarin and VX at the other, Jean Pascal Zanders, a chemical weapons expert at the European Union Institute for Security Studies, wrote in a blog post Friday.

“As the location and time of film footage cannot be ascertained, it is impossible to say whether the civilians were exposed to a mixture of toxic agents or different agents were released over different suburbs,” Zanders wrote. “Images of dead birds, cats and dogs tend to support impressions of a large volume of a rather fast-acting agent having been released in some locations.”

Dan Kaszeta, a chemical and biological warfare expert who runs the consulting firm Strongpoint Security, emphasized that without access to the scene, researchers will have a very difficult time determining exactly what happened.

“One issue is that you can’t really test for sarin gas, you test for chemicals that are released as it decomposes,” he said, adding that hospitals in Lebanon or Syria probably don’t have the state-of-the-art chemical laboratories to carry out the tests.

There’s a wide range of reported casualties, which only adds to the confusion and fuels the accusations of the pro-Assad camp that the opposition is exaggerating the numbers in hopes of inviting foreign military intervention that could help collapse the Syrian regime.

The Syrian Observatory for Human Rights, a pro-rebel monitoring group based in London, said on its Facebook page that it has documented the deaths of 136 people, including dozens of women and children, in the Ghouta attacks. A representative of the group said in an online interview that researchers were combing through hundreds of other names of purported victims and expects the final casualty figures to increase.

State Department officials say they’ve received reports that between 1,000 and 1,800 people were killed, figures that have been put forth by opposition figures.